1. Field of the Invention
The present invention relates to improvements in the chemical machining of silica, glass and like materials and, more particularly, to new and improved methods of precisely machining relatively deep cuts in silica, glass and the like at relatively high speeds.
2. Description of the Prior Art
Prior to selectively etching portions of objects or thin films of silica or glass, an etching mask is formed on the surface of the object or film to delineate the portion of the surface thereof to be contacted by etchant. Etchants commonly used for silica and glass are aqueous acid fluoride solutions. Effective etching masks must be formed from maskant materials which are highly resistant to these etchants. In addition, it is desirable that the maskant be highly adherent to those portions of the surface of the object or film which are to be protected from reaction with the etchant. Various photoresist materials, well known in the art, are effective maskants where the depth of etch required is on the order of 10,000 angstroms or less. This is typically the case in microelectronic circuit processing. However, in chemical machining where the depth of the etch in silica or glass is required to be on the order of one mil or more, the use of currently available photoresist materials is not practical. Even when fairly concentrated acid fluoride etchants are used, the rate of removal of silica or glass is less than 100 microinches per minute at room temperature. Over an extended etching period, the adhesion of photoresists to the silica or glass surfaces to be protected is impaired and, eventually, the photoresist material itself is lifted and washed away. Thus, for the relatively long etching times associated with chemical machining of silica and glass, the common practice has been to use maskants of wax although mixtures of wax and asphalt have occasionally been used. These materials are known to be highly resistant to acid fluoride etchants. For some applications, the wax maskant is protected by a metal template. This configuration is equally well described by calling it one in which an etch-resistant metal mask is held in place on and sealed to the appropriate surface regions of the silica or glass object by wax. Waxes usable as maskants (or seals) include peraffin waxes, chlorinated and unchlorinated glycidal esters of fatty acids, mixtures of these waxes, or a mixture of one or more of these waxes with asphalt. However, these materials have not proved to be effective maskants for chemical machining due to their relatively poor adhesion to silica and glass, particularly over relatively long etching times.
In one application of the chemical machining processes discussed herein, a portion of an article of quartz cut from a single crystal is required to be machined from an original thickness of about 30 mils to a finished thickness of from 1.5 to 2 mils. Precision of machining is required since the article is used in instruments such as accelerometers. Its physical dimensions and mechanical characteristics are therefore required to be within close tolerances of predetermined values. In this application, the machined region of the quartz forms a flexure for the accelerometer. Forming such a flexure by abrasive cutting would leave undesirable defects in the surface of the quartz.
One problem encountered when the prior art method of chemical machining is used, such as in forming a thin flexure region in quartz as discussed above, is lateral etching or undercut beneath the maskant. A relatively long tapering undercut results from seepage of etchant between the poorly adhering maskant and the quartz. It is not uncommon for the maximum lateral penetration of the etchant, and consequent removal of material under the maskant, to be as much as four times greater than the depth of etch. Relatively severe undercut occurs even when wax maskant is heated and reflowed one or more times between intervals of chemical machining. For some critical applications, parts are completely cleaned and recoated with fresh wax several times during intervals in the chemical machining to attain a reduced level of undercut. It has been determined that the etchant must be relatively dilute, i.e., less than about 50% by weight of hydrogen fluoride, to keep the undercut within the four to one range mentioned above. However, the more dilute the etchant, the more time is required for chemical machining.
Some prior art processes use very long chemical machining periods in a static etchant which is a mixture of one part of about 49% (by weight) hydrofluoric acid in an equal volume of cationic or non-ionic wetting agent. The wetting agent has an affinity for wax and tends to provide some sealing between the maskant and the silica or glass surface. With little or no agitation of the etchant, the use of the wetting agent reduces undercut to about two units of lateral etch for each unit of etching depth. However, for chemical machining the flexure region mentioned above, where about 14 mils of quartz are required to be removed from each side of the quartz blank, the exposure time required in the etchant is as much as 50 hours. This is due to the lack of agitation and the relatively low concentration of hydrogen fluoride in the etchant. Agitation cannot be used without destroying the tenuous film formed by the wetting agent and wax.
A need exists for methods of chemically machining silica and glass which are more rapid than the prior art methods and which give a more precise result by reducing undercut.